1. Field of the Invention
This invention relates to a brake system for automotive vehicles; more specifically, the invention relates to an anti-skid control system that prevents driveability impairment due to wheel locking during a braking operation.
2. Description of the Related Art
In a usual brake system for a vehicle, a master cylinder connected to a brake pedal, a wheel cylinder provided in a brake mechanism for each wheel and a brake fluid reservoir are connected by conduits. An anti-skid control system used in this type of the braking system is described, for example, in Japanese Patent Laid-Open publication No. Sho. 49-32494. In this example, a flow-in valve is installed in a conduit connecting a master cylinder and a wheel cylinder while a flow-out valve is installed in a conduit connecting the wheel cylinder and a reservoir. The brake fluid pressure in the wheel cylinder can be increased and decreased by controlling the flow-in and the flow-out valves. The brake fluid released from the wheel cylinder and reserved in the reservoir is returned to a conduit connecting the master cylinder and the flow-in valve by operation of a pump.
In this type of system, there is a drawback that the system gives a driver an unnatural feeling during the braking operation. This is because the brake pedal is pushed back by the brake fluid when the pressure in the wheel cylinder is decreased (this is commonly known as a "kick-back" phenomenon), or the brake pedal is unnaturally depressed when the brake fluid pressure in the wheel cylinder is increased.
To solve this problem, Japanese Laid-Open Patent publication No. Sho. 61-202965 discloses an anti-skid control system in which the pump is connected to the wheel cylinder to supply the brake fluid thereto during anti-skid operation. Thus, the driver's feeling during the braking operation is improved.
However, in the anti-skid control system shown in the aforementioned document, three control valves to switch the brake fluid on or off are required for a front-right wheel. This means six valves in total are necessary for two front wheels, resulting in a bulky and costly system.